The induction of a lymphopenic state frequently results in the development of organ-specific autoimmunity which is mediated by CD4+ T lymphocytes. Regulatory CD4+ T cells are selectively depleted by those procedures used to induce the lymphopenia. One implication of these findings is that autoreactive T cells which are capable of inducing autoimmune diseases exist in normal adult animals, but are maintained in a dormant or inactive state due to the suppressive functions of these regulatory T cells. We have demonstrated that the regulatory T cells can be easily identified in normal lymphoid tissues by co-expression of CD4 and the interleukin-2 receptor alpha chain (CD25). Transfer of CD4+ CD25- T cells to immunoincompetent mice results in the development of autoimmune disease which can be prevented by co-transfer of CD4+CD25+ cells. Our recent studies have focused on defining the mechanism of action of the CD4+CD25+ in vitro. CD4+CD25+ T cells are completely anergic to stimulation via their T cell receptor due to an inability to produce IL-2. When mixed with CD4+CD25- cells, they suppress proliferation by blocking transcription of the IL-2 gene in the CD25- population. The suppression was not cytokine mediated, required activation of the CD25+ T cells, and cell contact between suppressors and responders. The suppressor cells do not inhibit the generation of costimulatory molecules on antigen presenting cells, nor does excess costimulation abrogate suppression. The CD4+CD25+ population does not contain memory or conventionally activated T cells. CD4+CD25+ T cells isolated from TCR transgenic (Tg) mice inhibited responses of CD4+CD25- Tg T cells to the same antigen, but also inhibited the antigen-specific responses of Tg cells specfic for a distinct Ag. Suppression required that both peptide/MHC complexes be present in the same culture, but the peptides could be presented by two distinct populations of presenting cells. When CD4+CD25+ T cells were cultured with anti-CD3 and IL-2, they expanded, remained anergic, and in the absence of restimulation via their TCR, suppressed antigen-specific responses of CD4+CD25- T cells from different TCR Tg mice. These data demonstrate that CD4+CD25+ T cells require activation via their TCR to become suppressive, but once activated, their suppressor effector function is completely non-specific. One of the major problems is determining the specificity of the regulatory T cells is that the target antigen specificity of the autoimmune effector population is also unknown. We have demonstrated that post-day 3 thymectomy autoimmune gastritis in BALB/c mice is mediated by CD4+ T cells that recognize the H/K ATPase, the proton pump, of gastric parietal cells. We have further defined the fine specificity of the pathogenic H/K ATPase reactive population and isolated T cell clones that recognize different peptide epitopes on the alpha chain. Tg mice expressing the TCR from one of these clones were generated. Tg+ TCR expressing cells were positively selected in the thymus and peripheral T cells expressed the Tg TCR. Almost all Tg+ mice demonstrated severe gastric pathology. Gastric infiltrates were seen as early as three weeks of age. The Tg+ T cells appeared to have been selectively activated within the gastric lymph node as evidenced by a marked increase in CD69 expression which was not present on Tg+ cells in other lymphoid tissues. Very low numbers of T cells from the Tg mice could transfer very severe gastritis to immunoincompetent recipient, but not to normal recipients. This Tg strain should be an ideal model for our future studies on the mechanism of CD4+CD25+ suppressor function in vivo.